(1) Field of the Invention
The present invention relates to an image forming apparatus in which a printing medium is supported and conveyed by a conveyer and transfer belt which abuts the transfer area surface of the rotationally driven image bearer (photosensitive drum), with a predetermined positional relationship so that the toner image formed on the image bearer is transferred to and reproduced on the printing medium. More detailedly, the present invention relates to an image forming apparatus in which the image bearer is controlled so that it stops with a predetermined positional relationship when it is stopped to thereby become ready for subsequent image formation with precision.
(2) Description of the Prior Art
Conventionally, color image forming apparatus have been known in which a color image is transferred as image data of YMCK components to the recording portion whereby the color image is reproduced by forming color separations of the image and successively superimposing them one over another. So, color image forming apparatus of this type suffer from the problem in that a correct color image cannot be reproduced if each color separation cannot be exactly laid over the others. Therefore, how this problem is handled is the key to the technical development of this field.
In the image forming apparatus having a large number of parts, each part has small variations in precision. Further, individual image forming apparatus present variations in the assembly accuracy when these parts are assembled.
To deal with this, a configuration has been known conventionally, which forms trial color-separated pattern images and checks the positional relationship between these color separated pattern images to perform registration adjustment to image forming positions for individual component color image formations (c.f. Japanese Patent Application Laid-Open Sho 63 No. 286864).
Though it was possible to compensate for xe2x80x98color misregistrationxe2x80x99 due to xe2x80x98deviations of the start-position of writing of individual imagesxe2x80x99 by the execution of the aforementioned registration adjustment, it was impossible to correct the color misregistration attributed to the irregular speed variations of the photosensitive drums which are caused by periodic driving irregularities of the drive system, of drive gears etc. for driving the photosensitive drums.
Actually, the image forming apparatus of this kind has conventionally suffered from periodic driving unevenness in different recording portions. Due to the occurrence of the periodic driving unevenness in individual recording portions, the image forming apparatus has the problem of color misregistration arising when the images recorded with different coloring matters are sequentially superimposed one over another to reproduce a color image, making it impossible to reproduce a correct color image.
To deal with this, in the conventional color image forming apparatus, in order to synchronize the behavior of each photosensitive drum due to the driving unevenness when the images formed on the photosensitive drum in individual recording portions are transferred at in their transfer stations, arrangement of the image forming system has been manipulated in such a geometry that the distance (time) from the position of image recording to one photosensitive drum to its transfer position is set equal to N times (N is an integer) of the cycle period of the driving unevenness of the driving mechanism (see Japanese Patent Application Laid-Open Sho 62 No.129873 and Japanese Patent Application Laid-Open Sho 63 No.11965).
FIG. 1 shows a configuration of image forming portions and a conveyer and transfer belt and thereabout for transferring the images formed in the individual image forming portions in a conventional color image forming apparatus using the above technique.
In FIG. 1, photosensitive drums 322a, 322b, 322c and 322d, constitute the recording portions for black, cyan, magenta and yellow, from the right to left. The images of different colors formed on these photosensitive drums 322a to 322d are transferred, sequentially from black, onto the printing medium supported and conveyed by conveyer and transfer belt 316, at the transfer areas A where photosensitive drums 322a to 322d are located close to conveyer and transfer belt 316.
Here, photosensitive drums 322a to 322d are adapted to start rotating simultaneously and are attached in such an arrangement that the behavior of rotational driving unevenness is in phase with each other.
As an illustrative example, the driving gears (not shown) are fitted on the shafts of photosensitive drums 322a to 322d so that certain references (for example, keyhole shaped holes ha shown in FIG. 1) indicating the phase of the driving unevenness are oriented in the same direction. By this arrangement, photosensitive drums 322a to 322d will rotate with their driving unevenness always in phase if they are started simultaneously.
Therefore, when the distance Lxe2x80x2 between transfer areas A as to photosensitive drums 322a to 322d is set so that
xe2x80x83Lxe2x80x2=Nxcfx80d . . . (N is an integer)
where d is the diameter of each photosensitive drum 322a to 322d, in the image transfer process at each transfer area A of the four photosensitive drums 322a to 322d arranged in parallel to each other, images of different colors will be sequentially superimposed one over another with their behavior of the driving unevenness always being harmonized with respect to the printing medium. As result, it is possible to eliminate color misregistration due to driving unevenness.
However, in an image forming apparatus having a large number of parts, each part has small variations in precision. Or, individual image forming apparatus present variations in the assembly accuracy when these parts are assembled. Further, in the above configuration, the recording portions arranged in parallel to each other have to be arranged with their distances set in conformity with the periodic driving variations. So, when the recording portions are set with their distance from one to the next in conformity with the periodic driving variations, even with an integer N equal to 1, the distance at least needs to be equal to the circumference of the photosensitive drum. As a result, the image forming apparatus itself becomes bulky, in contrast to the user""s demands for downsizing. In particular, this problem is markedly significant in an image forming apparatus having four photosensitive drums of component colors Y, M, C and Bk, as stated above.
In the conventional apparatus shown in FIG. 1, in order to reduce the friction between the photosensitive drums and the conveyer and transfer belt as low as possible, photosensitive drums 322a to 322d start and stop rotating simultaneously. Further, in order to suppress the appearance of the driving unevenness derived from the driving system of the photosensitive drums, the start-position or behavior of driving unevenness in each photosensitive drum relative to conveyer and transfer belt 316 is kept in phase with that of the others so that photosensitive drums 322a to 322d start and stop rotating from their reference positions keeping the positional relationship between the drums and the conveyer and transfer belt 316 constant.
However, it is impossible to start and stop the movements of photosensitive drums 322a to 322d and conveyer and transfer belt 316 for the transfer operation, completely in synchronism. The time lag of the rotation and stoppage will impart a large contact friction to the delicate surfaces of the photosensitive drums around their reference positions. Thus, repetitions of the contact friction promotes damage to the surfaces of the photosensitive drums around the reference positions and consequently, a duplicated image degrades at the corresponding position, so the photosensitive drums have to be replaced in spite of their partial damage.
In connection with this, the damage around the reference position will build up with the passage of time and gradually present periodic image unevenness in the transferred image. So it is difficult to distinguish the defect from that from the aforementioned driving unevenness. In particular, it was very difficult to determine the cause of image unevenness if irregularities appear at the position corresponding to the aforementioned reference position of the driving unevenness.
It is therefore an object of the present invention to provide an image forming apparatus which can produce a correct reproduction of a color image by adaptively managing the periodic driving variations in the recording portions arranged in parallel to each other and which is compact in size having a minimum footprint for installation in office environments.
It is another object of the invention to provide an image forming apparatus which can suppress damage to the photosensitive drum surfaces, has improved efficiency and is economical, as well as being excellent in maintainability with respect to image unevenness.
In order to achieve the above objects, the present invention is configured as follows:
In accordance with the first aspect of the invention, an image forming apparatus wherein images formed on the surfaces of multiple image bearers arranged in parallel to each other and rotationally driven are sequentially transferred in a superimposed manner to a printing medium conveyed by a printing medium-conveyance unit at transfer stations each set up for individual image bearers, is characterized in that the distance between adjoining transfer stations is set shorter than the circumference of the image bearer and each image bearer is rotationally driven such that phase of rotational driving unevenness of each image bearer is shifted for compensating for the shortening of the distance between transfer stations so that periodic, rotational driving unevenness of each image bearer produces the same variation with respect to the printing medium passing through the transfer station.
In accordance with the second aspect of the invention, the image forming apparatus having the above first aspect is characterized in that each of the image bearers or each of the rotating members rotating in harmony with the rotation of the image bearer has a reference mark which enables identification of the phase of periodic driving unevenness of the image bearer, further comprising a controller for controlling rotational driving of all the image bearers based on the reference marks.
In accordance with the third aspect of the invention, the image forming apparatus having the above second aspect is characterized in that the controller stops rotations of individual image bearers at their respective stop-positions based on the reference marks and starts rotations of all the image bearers simultaneously.
In accordance with the fourth aspect of the invention, the image forming apparatus having the above third aspect, further comprises: detectors for detecting the reference marks, wherein each detector is arranged in the same geometry with respect to the corresponding transfer station, and is characterized in that the controller detects the reference mark of one image bearer as a reference for positioning and stops the reference image bearer at a point when a predetermined margin period of time has elapsed after the detection of the reference mark of the reference image bearer, and detects the reference marks of the other image bearers so as to determine the amounts of correction by comparing the time at which each detector detects its reference mark with the time at which the reference mark of the reference image bearer is detected, and stops each image bearer at a point when the sum of the predetermined margin period of time and the amount of correction has elapsed.
In accordance with the fifth aspect of the invention, the image forming apparatus having the above third or fourth aspect is characterized in that each image bear is provided with an individual driver source for driving its rotation independently from the others and each driver source uses a stepping motor or servomotor.
In accordance with the sixth aspect of the invention, the image forming apparatus having the above third through fifth aspect is characterized in that a stop-position adjuster for adjusting the stop-position of each image bearer is provided separately.
In accordance with the seventh aspect of the invention, an image forming apparatus wherein images formed on the surfaces of multiple image bearers arranged in parallel to each other and rotationally driven are sequentially transferred in a superimposed manner to a printing medium conveyed by a printing medium-conveyance unit at transfer stations each set up for individual image bearers, is characterized in that the distance between adjoining transfer stations is set shorter than the circumference of the image bearer and each image bearer is stopped such that phase of rotational driving unevenness of each image bearer is shifted for compensating for the shortening of the distance between transfer stations so that periodic, rotational driving unevenness of each image bearer produces the same variation with respect to the printing medium passing through the transfer station, and a controller is provided which controls each image bearer so that each image bearer will stop at the predetermined stop-position when there occurs a factor that varies the stop-positions of the image bearers.
In accordance with the eighth aspect of the invention, the image forming apparatus having the above seventh aspect is characterized in that the factor that varies the stop-positions of the image bearers is the case where the power to the apparatus is activated.
In accordance with the ninth aspect of the invention, the image forming apparatus having the above seventh aspect is characterized in that the factor that varies the stop-positions of the image bearers is the periodic check of the apparatus.
In accordance with the tenth aspect of the invention, the image forming apparatus having the above seventh aspect is characterized in that the factor that varies the stop-positions of the image bearers is the case where an abnormally fed printing medium within the apparatus is removed.
In accordance with the eleventh aspect of the invention, the image forming apparatus having the above seventh aspect is characterized in that the factor that varies the stop-positions of the image bearers is the case where the printing medium-conveyance unit has been separated and returned with respect to the image bearer surfaces.
In accordance with the twelfth aspect of the invention, the image forming apparatus having the above seventh aspect is characterized in that the factor that varies the stop-positions of the image bearers is the case where the predetermined number of image recording operation has been performed.
In accordance with the thirteenth aspect of the invention, the image forming apparatus having the above seventh aspect is characterized in that image formation is performed in a mode which uses at least one of a plurality of recording portions, and the controller controls so that the image bearer in each recording portion stops at the predetermined stop-position after recording of an image using at least one of a plurality of recording portions.
In accordance with the fourteenth aspect of the invention, the image forming apparatus having the above thirteenth aspect is characterized in that the controller controls so that the image bearer in each recording portion stops at the predetermined stop-position after recording of an image using the recording portion for image recording of a black developer.
In accordance with the fifteenth aspect of the invention, the image forming apparatus having the above seventh aspect is characterized in that the controller controls so that the image bearer in each recording portion stops at the predetermined stop-position with the printing medium-conveyance unit retracted from the image bearers.
In accordance with the sixteenth aspect of the invention, an image forming apparatus comprises: a rotationally driven image bearer; and a printing medium-conveyance unit abutted against the image bearer, wherein a developer image formed on the image bearer is transferred to a printing medium by passing the printing medium through the nip between the printing medium-conveyance unit and the image bearer, and is characterized in that a first abutment position on the surface of the image bearer against the printing medium-conveyance unit when the image bearer starts rotating differs from a second abutment position on the surface of the image bearer against the printing medium-conveyance unit when the image bearer stops rotating.
In accordance with the seventeenth aspect of the invention, an image forming apparatus comprises: a multiple number of rotationally driven image bearers; and a printing medium-conveyance unit abutted against the image bearers forming transfer stations, wherein developer images formed on the image bearers are transferred to a printing medium passing through the transfer stations so that the images are sequentially superimposed, and is characterized in that each image bearer is rotationally driven such that phase of rotational driving unevenness of each image bearer is shifted from others so that periodic, rotational driving unevenness of the image bearer produces the same variation as that of the others with respect to the printing medium passing through the transfer stations, and a first abutment position on the surface of each image bearer against the printing medium-conveyance unit when the image bearer starts rotating differs from a second abutment position on the surface of the image bearer against the printing medium-conveyance unit when the image bearer stops rotating.
In accordance with the eighteenth aspect of the invention, the image forming apparatus having the above seventeenth aspect is characterized in that the first abutment position of the image bearer of which the surface is likely to be most severely damaged among all the image bearers is set different from the second abutment position while the rest image bearers are stopped with their phases shifted from each other in the predetermined relationship.
In accordance with the nineteenth aspect of the invention, the image forming apparatus having any one of the above sixteenth through eighteenth aspect is characterized in that the first and second abutment positions are altered every predetermined number of stops or starts of driving of the plural image bearers.
In accordance with the twentieth aspect of the invention, an image forming apparatus comprises: a rotationally driven image bearer; and a printing medium-conveyance unit abutted against the image bearer, wherein a developer image formed on the image bearer is transferred to a printing medium by passing the printing medium through the nip between the printing medium-conveyance unit and the image bearer, and a controller is provided which controls the rotation of the image bearer in such a manner that the abutment position on the surface of the image bearer against the printing medium-conveyance unit when the image bearer starts rotating is altered every time the predetermined number of stops or starts of driving of the image bearer is reached.
In accordance with the twenty-first aspect of the invention, an image forming apparatus comprises:
a first controller having a sensor which detects a reference mark rotating in harmony with the rotation of an image bearer and controlling the image forming process for the image bearer based on the detected result from the sensor; and
a second controller controlling the stop-position of the image bearer based on the detected result from the sensor so that the reference mark is positioned in the predetermined relationship with respect to the sensor.
In accordance with the twenty-second aspect of the invention, the image forming apparatus having the above twenty-first aspect is characterized in that the reference mark is attached on the image bearer surface.
In accordance with the twenty-third aspect of the invention, the image forming apparatus having the above twenty-second aspect is characterized in that the reference mark is attached between a conductive supporting member and photoconductive layer constituting the image bearer.
In accordance with the twenty-fourth aspect of the invention, the image forming apparatus having the above twenty-first aspect is characterized in that the reference mark is attached on a drive transmission member for transmitting a rotational driving force to the image bearer.
In accordance with the twenty-fifth aspect of the invention, the image forming apparatus having the above twenty-first aspect, further comprises a controller for switching the stop-position of the image bearer, periodically.
In accordance with the twenty-sixth aspect of the invention, the image forming apparatus having the above twenty-first aspect, further comprises a driving mechanism for supporting the image bearer and transmitting a rotational driving force to the image bearer, wherein the image bearer is supported in the predetermined relationship with respect to the driving mechanism.
In accordance with the twenty-seventh aspect of the invention, the image forming apparatus having the above twenty-sixth aspect, further comprises a checking member for checking whether the sensor can detects the reference mark attached on the image bearer when a new image bearer is supported with respect to the driving mechanism.
In accordance with the twenty-eighth aspect of the invention, the image forming having the above twenty-sixth or twenty-seventh aspect is characterized in that when the sensor detects the reference mark attached to the image bearer, the image bearer stop-position control by the second controller is performed whereas if the sensor cannot detect the reference mark, the image bearer stop-position control by the second controller will not be performed.
According to the invention defined by the first aspect, since the distance between the adjacent transfer stations is set shorter than the circumference of the image bearer, it is possible to make the apparatus compact as compared to the conventional configuration where N=1. Further, the phase angle of each image bearer is rotated and shifted out of phase from the others for compensating for the above shortening, so that periodic, rotational driving unevenness of each image bearer produces the same variation with respect to the printing medium passing through the successive transfer stations. Therefore, the images formed on different image bearers can be sequentially superimposed under the same condition without having any influence of periodic driving unevenness of the image bearers. Hence, the final output image can be reproduced correctly without color misregistration.
According to the invention defined by the second aspect, the aspect of the first invention can be easily attainted by controlling the driving of each image bearer based on its reference mark (any mark will make do as a reference such as a cutout in the gear shaft or the like) which enables identification of the phase of periodic driving unevenness of the image bearer.
According to the invention defined by the third aspect, the following advantage can be obtained. In general, all the image bearers are started to rotate at the same time and stopped at the same time. This is to prevent the image bearers from being damaged by their friction with the printing medium-conveyance unit (conveyer and transfer belt). Even if the rotation of each image bearer is started and stopped at different timing from the others, there is no concern of the image bearers being damaged if the printing medium-conveyance unit is separated from the image bearers. However, this method entails a time loss. Therefore, since the phase of the rotational stop-position (which means the start-position for rotation) of each image bearer is shifted as above, it is possible to realize the driving method described in the first aspect in a simple manner.
According to the invention defined by the fourth aspect, each image bearer is stopped at a position after a lapse of a period of time containing a predetermined margin period of time, instead of stopping it immediately after the detection of the reference mark. Therefore, the predetermined stop-state can be realized in a marginally minimized time (with a marginally minimized rotary angle).
In the above case, suppose that no margin period of time is reserved, if the amount of correction for a certain image bearer with respect to the reference image bearer is positive, the image bearer may be stopped after an extra rotation corresponding to the amount of correction. However, when the amount of correction is negative, the image bearer goes beyond the position to be stopped. So to stop the image bearer at the correct position, the image bearer need to be rotated one more revolution. As already stated, in the conventional configuration, all the image bearers are started to rotate at the same time and stopped at the same time while the printing medium-conveyance unit is continuously (other than the jammed paper removal) put in proximity to the image bearers, so that such a large rotation will produce marked damage to the surfaces of the image bearers. Thus, the above configuration is able to set the apparatus into the stand-by state which enables an ideal recording of an image, in a short period while suppressing damage to the surface of each image bearer, making it possible to perform smooth recording of a subsequent image.
According to the invention defined by the fifth aspect, use of a simple configuration positively enables each image bearer to be stopped in a suitable state.
According to the invention defined by the sixth aspect, the positional relationship between the adjoining transfer stations can be corrected by adjusting the stop-position of the image bearer facing the transfer station downstream. Similarly, even if the positional relationship between the detectors for detecting the reference marks is disordered, it is possible to adjust and correct the stop-position of the image bearer for which the detector is displaced, in a similar manner.
According to the invention defined by the seventh aspect, if there occurs a factor that varies the stop-position of the image bearer in each recording portion, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position. Therefore, the image bearer in each recording portion is stopped and set in the appropriate stop-position before the recording of an image is started so that for subsequent operations the image of each component color can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image.
According to the invention defined by the eighth aspect, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position when the power to the apparatus is activated. Therefore, the image bearer in each recording portion is always stopped and set in the appropriate stop-position before recording of an image is permitted so that for subsequent operations the image of each component color can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image. Further, the apparatus can start recording of an image in a suitable condition as soon as it becomes prepared for recording.
According to the invention defined by the ninth aspect, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position after the periodic check (after parts replacement, unit adjustment, etc.). Therefore, whenever the periodic check such as parts replacement, unit adjustment or the like, which is highly likely to cause variations in the stop-positions, has been done, the image bearer in each recording portion is stopped and set in the appropriate stop-position so that for subsequent operations the image of each component color can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image. Further, the apparatus can start recording of an image in a suitable condition as soon as it becomes prepared for recording.
According to the invention defined by the tenth aspect, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position after removal of abnormally fed printing medium. Therefore, it is possible to configure the driving mechanism so as to be temporarily released to facilitate easy removal of the printing medium that caused paper jamming, from the conveyance path without giving damage to the image bearer surfaces, and after the removal, the image bearer in each recording portion is always stopped and set in the appropriate stop-position so that for subsequent operations the image of each component color can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image.
According to the invention defined by the eleventh aspect, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position after the release of the printing medium-conveyance unit. Therefore it is possible to provide a configuration which permits easy removal of the printing medium that caused paper jamming from the conveyance path, and after the removal, the image bearer in each recording portion is always stopped and set in the appropriate stop-position so that for subsequent operations the image of each component color can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image.
According to the invention defined by the twelfth aspect, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position every predetermined times of image recording. Therefore it is possible to minimize (correct) the misregistration, due to periodic driving unevenness of the image bearer in each recording portion, increasing as recording proceeds. As a result, the image of each component color can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image.
According to the invention defined by the thirteenth aspect, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position after image formation has been performed with at least one of a plurality of recording portions stopped. Therefore, after image recording using part of a plurality of recording portions, the image bearer in each recording portion is stopped and set in the predetermined stop-position so that for subsequent operations the image of each color developer can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image.
According to the invention defined by the fourteenth aspect, the image bearer in each recording portion is controlled so as to be stopped at the predetermined stop-position after image formation of black developer has been performed. Therefore, after image recording using part (the black image recording portion) of the multiple recording portions, the image bearer in each recording portion is stopped and set in the predetermined stop-position so that for subsequent operations the image of each color developer can be exactly superimposed over the others without being affected by periodic, driving unevenness, thus making it possible to reproduce a correct color image.
According to the invention defined by the fifteenth aspect, since the printing medium-conveyance unit is retracted from the image bearers in the recording portions when the image bearer in each recording portion is controlled to stop at the predetermined stop-position, the printing medium-conveyance unit is separated from the image bearer surfaces so as to minimize the load acting on the image bearer surface of each recording portion, thus making it possible to make sure the stop-position of each image bearer. This configuration also contributes to prevention of damage (scratching) to the image bearer surfaces.
According to the invention defined by the sixteenth aspect, since regardless of monochrome copy or color copy, the abutment position on the surface of the image bearer against the printing medium-conveyance unit when the image bearer starts rotating differs from the abutment position when the image bearer stops rotating, rubbing of the surface of the image bearer with the printing medium-conveyance unit arising due to the time lag at the start or stop of driving will not concentrate at one point. Thus, damage to the image bearer surface due to its contact with the printing medium-conveyance unit will not concentrate at a local area. Therefore, it is possible to prevent marked, local performance degradation of the image bearer.
Further, since the first abutment position and the second abutment position are set different when the image bearer is rotated, it is possible to efficiently change the abutment position.
According to the invention of the seventeenth aspect, each image bearer is rotated with its driving unevenness shifted out of phase from the others so that periodic, rotational driving unevenness of each image bearer produces the same variation with respect to the printing medium passing through the successive transfer stations. Therefore, the images formed on different image bearers can be sequentially superimposed under the same condition without having any influence of periodic driving unevenness of the image bearers. Hence, the final output image can be reproduced correctly without color misregistration.
Since the abutment position on the surface of the image bearer against the printing medium-conveyance unit when the image bearer starts rotating differs from the abutment position when the image bearer stops rotating, rubbing of the surface of the image bearer with the printing medium-conveyance unit arising due to the time lag at the start or stop of driving will not concentrate at one point. Thus, damage to the image bearer surface due to its contact with the printing medium-conveyance unit will not concentrate at a local area. Hence, it is possible to prevent marked, local performance degradation of the image bearer.
Further, since the first abutment position and the second abutment position are set different when the image bearer is rotated, it is possible to efficiently change the abutment position.
According to the invention of the eighteenth aspect, among all the image bearers in the multiple recording portions arranged in series in the conveyed direction of the printing medium, the stop-position of the image bearer which is likely to be most severely degraded is varied while the rest image bearers are stopped with their rotational driving unevenness shifted from each other in the predetermined relationship. Therefore, the stop-positions of the rest image bearers are corrected in conformity with the image bearer having the most intensively degraded due to the difference of the image forming mode. Thus, damage to the image bearer exposed to the most harsh conditions can be made uniform by preventing it from being alone degraded too much. Accordingly, since each image bearer is degraded on the average, it is possible to reduce the number of maintenance.
According to the invention of the nineteenth aspect, the first and second abutment positions are altered every predetermined number of stops or starts of driving of the multiple image bearers. Therefore, it is possible to efficiently prevent damage to the image bear surfaces by selecting the predetermined number in its various usage conditions.
According to the invention of the twentieth aspect, since regardless of monochrome copy or color copy, the abutment position on the surface of the image bearer against the printing medium-conveyance unit can be altered every time the predetermined number of stops or starts of driving of the image bearer is reached, rubbing of the surfaces of the image bearer and the printing medium-conveyance unit arising due to the time lag at the start or stop of driving will not concentrate at local areas. Thus, damage to the image bearer surface due to its contact with the printing medium-conveyance unit will not concentrate at one point. Hence, it is possible to prevent marked, local performance degradation of the image bearer.
According to the invention defined by the twenty-first aspect, since both the first control means for controlling the image forming process and the second control means for controlling the stop-position of the image bearer are configured to perform their control based on the common sensor detecting the reference mark, the cost can be reduced without the need to provide a dedicated sensor for each. Further, since the image bearer is stopped at the predetermined position by directly detecting the reference mark which rotates in harmony with the rotation of the rotationally driven image bearer, it is possible to precisely stop the rotation of image bearer with the desired relationship taking into account driving unevenness as well as damage to the image bearer, etc.
According to the invention defined by the twenty-second aspect, since the reference mark which is attached on the surface of the rotationally driven image bearer is directly detected so as to stop the image bearer at the predetermined position, it is possible to precisely stop the image bearer every time, with the desired relationship.
According to the invention defined by the twenty-third aspect, since the reference mark is coated by the se photoconductive layer, it is less damaged. Since the position of attachment is not limited, it is possible to enhance the flexibility of the position of attachment of the sensor.
According to the invention defined by the twenty-fourth .aspect, since the reference mark which is attached on a drive transmission member for transmitting a rotational driving force to the image bearer is directly detected so as to stop the image bearer at the predetermined position, it is possible to precisely stop the image bearer every time, with the desired relationship.
According to the invention defined by the twenty-fifth aspect, since the stop-position of the image bearer is altered periodically every predetermined number of copiers and/or after a lapses of a predetermined period of time, the position of the image bearer in contact with other parts is changed periodically so that it is possible to prevent the image bearer surface from being damaged locally, and hence lengthen the life of the image bearer.
According to the invention defined by the twenty-sixth aspect, since the image bearer is supported in the predetermined relationship with respect to the driving mechanism and hence the behavior of periodic rotational driving unevenness of the rotationally driven image bearer will fall within the expected range, it is possible to take reliable countermeasures against the rotational driving unevenness.
According to the invention defined by the twenty-seventh aspect, it is possible to check whether the image bearer is supported in the predetermined relationship with the driving mechanism, while for subsequent operations the stop-position of the image bearer can be controlled keeping the predetermined relationship.
According to the invention defined by the twenty-eighth aspect, when designated image bearers are set with the predetermined positional relationship, the image bearers can be precisely stopped in the predetermined relationship. If the image bearers are attached in a wrong manner or a wrong image bearer other than that designated is placed, the image reproduction is continued to output an image whatever it image instead of completely stopping the machine, so as not to offend the user.